The importance of membranes for treatment of wastewater is growing rapidly. It is now well known that membrane processes can be used as an effective tertiary treatment of sewage and provide quality effluent. However, the capital and operating cost can be prohibitive. With the arrival of submerged membrane processes where the membrane modules are immersed in a large feed tank and filtrate is collected through suction applied to the filtrate side of the membrane or through gravity feed, membrane bioreactors combining biological and physical processes in one stage promise to be more compact, efficient and economic. Due to their versatility, the size of membrane bioreactors can range from household (such as septic tank systems) to the community and large-scale sewage treatment.
The success of a membrane filtration process largely depends on employing an effective and efficient membrane cleaning method. Commonly used physical cleaning methods include backwash (backpulse, backflush) using a liquid permeate or a gas or combination thereof, membrane surface scrubbing or scouring using a gas in the form of bubbles in a liquid. Typically, in gas scouring systems, a gas is injected, usually by means of a blower, into a liquid system where a membrane module is submerged to form gas bubbles. The bubbles so formed then travel upwards to scrub the membrane surface to remove the fouling substances formed on the membrane surface. The shear force produced largely relies on the initial gas bubble velocity, bubble size and the resultant of forces applied to the bubbles. To enhance the scrubbing effect, more gas has to be supplied. However, this method consumes large amounts of energy. Moreover, in an environment of high concentration of solids, the gas distribution system may gradually become blocked by dehydrated solids or simply be blocked when the gas flow accidentally ceases.
Furthermore, in an environment of high concentration of solids, the solid concentration polarisation near the membrane surface becomes significant during filtration where clean filtrate passes through membrane and a higher solid-content retentate is left, leading to an increased membrane resistance. Some of these problems have been addressed by the use of two-phase flow to clean the membrane.
Cyclic aeration systems which provide gas bubbles on a cyclic basis are claimed to reduce energy consumption while still providing sufficient gas to effectively scrub the membrane surfaces. In order to provide for such cyclic operation, such systems normally require complex valve arrangements and control devices which tend to increase initial system cost and ongoing maintenance costs of the complex valve and switching arrangements required. Cyclic frequency is also limited by mechanical valve functioning in large systems. Moreover, cyclic aeration has been found to not effectively refresh the membrane surface.
It would be desirable to provide an energy efficient operation of the scouring process without the need to control the operation by means of complex valve switching etc. It is also preferable to provide a two-phase liquid gas flow past the membrane surfaces to provide a more effective scouring process while minimising energy requirements for such a cleaning process.